Frequency control system



June 21, 1949.

J. R. BOYKIN FREQUENCY CONTROL SYSTEM 2 Sheets-Sheet 1. a HI- V INVENTOR.fahn EBayK/n ATTOR Y Filed Jan. 22, 1946 WITNESSES: aim/W $0. 4

June 21, 1949- J. R. BOYKIN 2,473,853

FREQUENCY CONTROL SYSTEM Filed Jan. 22, 1946 v 2 Sheets-Sheet 2WITNESSES: INVENTOR 477;/% jig/7n EBoyK/n.

ATTORNEY Patented June 21, 1949 FREQUENCY CONTROL SYSTEM John R. Boykin,Baltimore, Md., assignor to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Application January 22,1946, Serial No. 643,006

8 Claims.

My invention relates to frequency regulating devices, and in particularrelates to devices for maintaining the average frequency of anoscillator generator, governed in periodicity by aninductance-capacitance network, equal at all times to the frequency of astandard-frequency source such as an oscillator controlled by apiezo-electric vibrator. This arrangement is particularly well adaptedto maintain the center frequency of an oscillation generator forfrequency-modulated Waves constantly equal to the frequency of astandard-frequency source. In the embodiment here used to illustrate theprinciples of my invention, it is applied through the agency of areactance tube which regulates the reactance of the frequencydetermining circuit of the regulated oscillator.

This invention is a modification of, and in some respects an improvementupon, the frequency measuring and regulating device described in mycopending application for measuring devices filed June 16, 1945, SerialNo. 599,841, now abandoned, and assigned to the Westinghouse ElectricCorporation.

One object of my invention is to provide an arrangement which shallproduce a direct-current voltage which is responsive to variations infrequency between two alternating-current sources.

Another object of my invention is to provide an arrangement whichemploys a direct-current voltage to regulate the reactance of areactance-tube.

Still another object of my invention is to provide an arrangement inwhich a direct-current voltage changes the control voltage on areactance tube incorporated in the frequency-determining circuit of anoscillation generator whenever the frequency of said generator deviatesfrom that of a second alternating-current source.

A still further object of my invention is to provide an arrangement inwhich the charge on a capacitor is altered whenever the frequency of onealternating-current source deviates from that of anotheralternating-current source.

Still another object of my invention is to provide a system in which thecharge on a capacitor is increased when the frequency of onealternating-current source becomes greater than that of a secondalternating-current source, and in which the said charge is decreasedwhen the frequency of the first-mentioned altemating-current sourcebecomes smaller than that of the second-mentioned alternating-currentsource.

Other objects of my invention will become apparent upon reading thefollowing description taken in connection with the drawings, in which:

Figure 1 is a schematic diagram illustrating one circuit embodying theprinciples of my invention, and

Figs. 2 to '7 are graphs of wave forms useful in describing the mode ofoperation of the circuit of Fig. 1.

Referring in detail to Fig. 1, reference numeral 5 represents analternating-current source, of which the frequency is to be regulatedand reference numeral 2 represents a standard frequency source such, forexample, as a generator controlled by a piezo-electric vibrator. Thesources I and 2 impress their output voltages through suitable phasedisplacing networks adapted to cause them to make the voltage impressedby one of them on a pair of mixer tubes 3 and 4 which are similar to thesimilarly numbered tubes in my above-mentioned copending application,wherein similar elements are designated by the same reference numerals.Thus the respective alternating-current sources I and 2 are respectivelyconnected through suitable coupling capacitors 2| and 22, to resistors23 and 24, of which one terminal is grounded. The nongrouncled terminalof resistor 23 is connected to one of the control grids 25 of a mixertube 3 and also to the control grid 26 of a mixer tube 4. The ungroundedterminal of the resistor 24 is similarly connected through a capacitor21 to the other control grid 28 of the mixer tube 3 and is directlyconnected through a resistor 29 to the other control grid 3| of themixer tube 4. The cathode of the mixer tube 3 is preferably groundedthrough a resistor 32 and the cathode of the mixer tube 4 is similarlygrounded through a resistor 33. A resistor 34 connects the control grid28 to ground and a capacitor 35 connects the control grid 3| to ground.The anodes of mixer tubes 3 and 4 are respectively connected throughresistors 36 and 31 to the positive terminal 33 of a suitabledirect-current voltage source having its negative terminal grounded. Thecathodes of the mixer tubes .3 and 4 are connected through suitableresistors 39, 4!, 42 and 43 to the positive terminal 38. Screen andsuppressor grids in the mixer tubes 3 and 4 may be respectivelyconnected through resistors, in accordance with well-known practice inthe radio art to the common terminals of resistors 39, 4|, 42, 43.

The output of the mixer tube 3 is connected through a capacitor 5| tothe control grid of a triode 52 provided with a serially-connectedresistor 53 between its cathode and ground, thereby providing a networkwhich functions as an amplifier. The anode of the tube 52 is connectedthrough a resistor St to the positive terminal 38 and the control gridis connected to ground through a resistor 55. The anode of the mixertube l is connected through a capacitor MA and a resistor 55A to anamplifier tube 52A having connections like tube 52 and indicated bycorresponding reference numerals with the suffix A.

As in the case of my above-described application, the output of themixer tubes 3 and produce voltages having a frequency equal to thedifference in frequency existing at any moment between the sources and2, the phase of the output voltage from mixer 4 leading by '90 theoutput voltage from mixer 3 when the frequency of source exceeds thatof. source 2, and lagging by 90 behind the voltage of the mixer 3 whenthe frequency of source l is less than that of source 2. voltage ofmixer 3, the'output voltage of mixer i will be represented by theportion of Fig. 3 to the left of the vertical line when the frequency ofsource l exceeds that of source 2, and will be represented by theportion of Fig. 3 to the right of the vertical line when the frequencyof source I is less than that of source 2.

The output of one of the above-mentioned mixers, for example, mixer 3 isconnected through a pair of series connected capacitors 66 and El,

to the input of an electron tube 62 having connections causing it to actas a multi-vibratr of the type commonly referred to as flip-flop. Thusthe capacitor 5! is shunted by a resistor 63 and connected to thecontrol electrode ti of the tube 62. The control electrode 6t and thecathode associated therewith at the tube 62 are grounded, respectively,through resistors 65 and G5. The anode associated with the control grid64 is connected through a resistor 61 to the positive terminal of adirect-current source having its negative terminal grounded. A secondanode in the tube 62 is connected through a second resistor 68 to thesame positive terminal, and is also connected to the control grid 64through a resistor 69. The cathode associated with the last-mentionedanode is connected directly to the cathode associated with control gridE i, and a control grid associated with the last-mentioned cathode isconnected to ground through a resistor H and to the anode which isassociated with control grid M by a resistor H2. The connections justdescribed will be recognized by those skilled in the art as causing thetube 62 to produce square topped output waves having the same phase asthe sinusoidal input voltage illustrated in Fi 2 as representing theoutput of the mixer 3. It is also within the purview of my invention tosubstitute for the multi-vibrator 62 any other form of device, such as asaturated amplifier, adapted to produce square topped waves whenimpressed with sinusoidal waves such as those of Fig. 2.

A pair of output capacitors l3 and T4 connect the respective anodesofthe tube 62 with two sets of diode electrodes in a tube 15, therebyimpressing on the respective anodes of said tube voltages which aredifferentials of the square topped output waves of opposite phaseproduced across the two sets of principal electrodes in the tube 52.Thus, Fig. 4 represents the sharply peaked pulses constitutingdifferential voltage from the anode associated with control electrode 64and Fig. 5 represents the oppositely poled pulses constituting thedifferential voltages derived from the other anode of the tube 62. Itwill be noted Thus, if Fig. 2 represents the output that since thesquare topped waves produced by the multi-vibrator 62 are in phase withthe sinusoidal waves in Fig. 2, the sharply peaked pulses in Figs. 4 and5 coincide in time with the instants when the sine voltage of Fig. 2passes through zero; and correspondingly these sharply peaked pulses arecoincident with the maximum and minimum points on the sinusoidal outputwave of Fig. 3 which it will be remembered is displaced in phase fromthat of Fig. 2, and which represents the output voltage of mixer t.

The output of mixer d is connected through a pair of resistors '56 andTi to the respective anodes of tube E5. The cathodes of the diodes intube iii are connected to each other through a pair of equal resistors78 and 79 whose midpoint is connected to the positive terminal of avoltage source having its negative terminal grounded. The result of theforegoing connections is that the sinusoidal output of mixer No. 4,which is represented by Fig. 3, has superposed upon it the pulsesrepresented by Figs. a and 5. The result of such superposition isrepresented in Figs. 6 and 7 Thus on the right-hand diode embodied intube '55, there will be superposed on the sinusoidal wave of Figure 3,the pulses represented by Fig. i. It will be remembered that theleft-hand portion of Fig. 3 represents the output voltage of mixer 4when the frequency source I is greater than that of source 2; andcorrespondingly the portion of Fig. 6 to the left of the vertical linerepresents the superimposition of the pulses of Fig. i on the left-handportion of Fig. 3. On the other hand, the right-hand portion of Fig. 3represents the output voltage of mixer 4 when the frequency of source iis less than that of source 2, and the right-hand portion of Fig. 6correspondingly represents the superimposition of the pulses of 4 on theright-hand portion of Fig. 3.

Fig. '7 represents the superimposition, in the left-hand diode of tube75, of the sinusoidal out-- put voltage of mixer i shown in Fig. 3 uponthe pulsed voltage of Fig. 5.

The horizontal lines near the top of Figs. 6 and '7 represent thecritical voltage required to cause current flow through the two diodescomprised in tube 15. It will be noted from the lefthand portion of Fig.6 that the pulses of Fig. 4 are always so timed that they subtract fromthe peak value of the sinusoidal wave as a result of which the voltageimpressed on the left-hand diode in tube 55 never exceeds theabove-mentioned critical voltage required to cause current flow throughtube 15 as long as the frequency of source I exceeds that of source 2.On the other hand, from the right-hand side of Fig. 6 corresponding tothe condition where the frequency of source 2 exceeds that of source I,the pulses are superposed on the sinusoidal wave so that they increasethe peak value of the resultant wave and cause it to rise above thevalue necessary to cause current flow through the right-hand diode oftube l5. In short, current will be conducted through the right-handdiode in tube 75 only when the frequency of source i is less than thatof source 2.

Now turning to Fig. 7 which represents conditions in the left-hand diodein tube 15, it will be noted from the left-hand half of Fig. 7corresponding to the condition when the frequency of source I exceedsthat of source 2 that the pulses are superposed on the peak value of thesinusoidal wave so that they exceed the value necessary to cause currentflow through the left-hand diode in tube I5 at times when the frequencyof source I exceeds that of source 2. On the other hand, from theright-hand side of Fig. '7, it is evident that the superposition of thepulses of Fig. 5 on the sinusoidal wave of Fig. 3 is never suflicient tocause current to flow through a left-hand diode in tube I5 when thefrequency of source I is less than that of source 2.

In short, current flows for brief intervals through the right-hand diodein tube when the frequency of source I is less than that of source 2,and flows through the left-hand diode intube 75 when the frequency ofsource I is greater than that of source 2.

The cathode of the left-hand diode in tube '15 is connected through acapacitor 8| in series with a resistor 82 to ground and the cathode ofthe right-hand diode in tube I5 is connected through a capacitor 83 anda series resistor 84 to ground.

The common terminal of the capacitor 8| and resistor 82 is connected tothe control electrode governing current flow between a first anode andcathode in an electron tube 85, and the common terminal of the capacitor83 and resistor 84 is connected to a second control electrode governingcurrent flow between a second anode and cathode in tube 85. The cathodesof the tube 85 are connected together and grounded through a resistor86. The first anode of the tube 85 is connected to a first cathode inthe electron tube 8?, and the second anode of the tube 85 is connectedto a second cathode in the tube 81. The anodes of the tube 81 areconnected together through a resistor 88 to the positive terminal of asource of direct-current voltage which has its negative terminalgrounded. These anodes are also connected to ground through a capacitor89. The two cathodes of the tube 8'! are connected to each other througha pair of serially connected equal resistors 9| and 92 and the midpointof these resistors is connected to the positive terminal of anotherdirect-current voltage source having its negative terminal grounded. Theanodes of the tube 81 are likewise connected through a resistor 93 tothe two cathodes of the tube 85. The tube 85 is so designed and suppliedwith voltages from the sources above mentioned-as to be biased to cutoff when neither of the diodes of tube I5 is conducting current, and tobedriven to saturation by the pulses impressed on its control selectrode when either of the diodes in tube I5 conducts the short pulsesof current described above. From this arrangement it follows that one orthe other of the anodes of tube 85 will conduct a definite quantity ofcurrent whenever a pulse is produced in the associated diode of tube 75.The diodes in tube 87 have a common bias and insure that the saturationvoltage of the two triodes comprised in tube 85 are equal.

An amplifier tube 98 has its anode connected to the positive terminal ofa direct-current voltage source, the negative terminal of which isgrounded, and has its cathode connected through a pair of seriallyrelated resistors 99 and I00 to the negative terminal of adirect-current voltage source IOI which has its positive terminalgrounded. A second amplifier tube I02 has its anode connected to thepositive terminal of a voltage source having its negative terminalgrounded and has its cathode connected to ground through a resistor I03and also connected through another resistor I04 to the negative terminalof the voltage source IOI.

A rectifier I05, which may be a diode, has

its cathode connected to the cathode of thetube 6 98. A capacitor 94 isconnected between the first anode of the tube and the anode of therectifier I05. A capacitor 95 is connected between the cathode of thetube 98 and ground.

A rectifier I08, which may be a diode, has its anode connected to thecommon terminal of the resistors 99 and I00. The cathode of therectifier I08 is connected through a capacitor 96 to the second anode ofthe tube 85. A capacitor 91 is connected between the anode of therectifier I09 and ground. The common terminal of capacitor 94 and therectifier I05 is likewise connected to the negative terminal or cathodeof a second rectifier I00 which may be a diode and which has itspositive terminal connected to the control grids of the tubes 98 andI02.

The common terminal of the capacitor 98 and the rectifier I00 isconnected to the positive terminal or anode of a rectifier I07, whichmay be a diode having its negative terminal connected to the controlgrids of the tubes 98 and I02. The control grids of the tubes 98 and I02are connected to ground through a capacitor I09.

The mode of operation of the circuit comprising tubes 85, 87, 98 and I02is as follows: When no current pulses are flowing through either of thediodes of tube I5, the capacitors 94 and will stand charged to somevoltage by current flowing through the resistors SI and 92, thecapacitors and 97, the four rectifiers above described, the capacitorI09, resistors 99, I00 and source I0 I. Now suppose a current pulse istransmitted thru the left-hand diode in tube '15 by reason of the factthat the frequency of source I has become greater than the frequency ofsource 2. The left-hand triode in tube 85 will be rendered conductive,thereby producing a momentary increase of voltage drop through theresistor 9| which will be a fixed amount because of the saturatedcondition of tube 85. The capacitor 90 will, therefore, discharge afixed quantity of current through the circuit comprising resistor 9|,capacitor I09 and rectifier I00. When the current pulse ceases flowingthrough the lefthand diode of tube I5, the grid of the left-hand triodeof tube 85 will return to its normal potential, thereby cutting offcurrent flow through tube 85 and resistor 9|. The voltage drop inresistor 9! will thus also return to its initial condition and thecharge displaced from capacitor 94 will flow back through rectifier I05,from ca pacitor 95 which is charged through resistors 99 and I90 andsource IOI. The result of the foregoing action has accordingly been todeposit a fixed and definite charge in the capacitor I09, therebyincreasing by a definite amount the negative potential of the grids oftubes 08 and I02 relative to ground.

In consequence of the fact that its control grid has been made morenegative, the current flow through the resistor I03 in the cathode leadof tube I02 will decrease similarly by a definite amount. The ungroundedterminal of the resistor I03 is connected to the control electrode of areactance tube and this decrease of positive potential on the controlelectrode will increase the equivalent reactance which that reactancetube introduces into the frequency determining circuit of thealternating-current source I, thereby decreasin the frequency of sourceI and causing it to approach more nearly that of standard source 2. Ifthis decrease of frequency of source I is not suificient to bring itinto equality with that of source 2, further pulses will be transmittedthrough the left-hand diode in tube I5, thus imposing a succession offurther negative charges on the capacitor I519 and making the controlelectrode of the reactance tube less and less positive until thefrequency of source 5 has been reduced to equality with that of source2.

On the other hand, if the frequency of source I drops below that ofsource 2 at any instant, a current pulse will flow through theright-hand diode in tube l5 making the associated control electrode oftube 85 more positive, and producing an outflow of charge of constantand definite amount from capacitor 9%. Capacitors 94 and 96 are madeequal to each other so that, just as has previously been described forthe case of capacitor 9d, a definite charge of constant amount will bedeposited on the capacitor H39. However, it will be noted that by reasonof the opposite polarities of the rectifiers Hi1 and it, the controlgrids of the tubes 93 and (92 will be made more positive by a definitevoltage by reason of the deposition of the charge last mentioned.Current flow through the tube I62 will, accordingly, make the voltagedrop through resistor '23 greater by a fixed amount and will therebydecrease the effective reactance of the reactance tube previouslymentioned, so that the frequency of the source 5 will increase. Further,current pulses will flow through the right-hand diode in tube '55 withconsequent further increase of positive potential on the controlelectrode of tube H12 until the frequency of source l is raised toequality with that of source 2. From the foregoing discussion, it willbe evident that the capacitances of the capacitors 3d and 96 may besubstantially smaller than the capacitances of the capacitors Q5, 91 andHill.

It will be noted that after a current pulse from the left-hand diode oftube 55 has caused the momentary discharge from capacitor 94 of adefinite quantity of electricity, a charge has been deposited incapacitor Hi9, thereby making the potential of the anode of rectifier m6less positive than it was before the transaction just mentionedoccurred. It will likewise be remembered that the initial discharge ofcapacitor oil occurred through rectifier Hi6 and capacitor I09 andconsequently the point at which in-fiow of charge ceased was dependentupon the potential of the anode of rectifier Hit relative to itscathode. Unless some compensation were introduced the amount of chargeflowing into capacitor 9 3 would thus be different with each succeedingpulse of current flowing through the left-hand diode of tube 15, andsuch compensation generally is desirable. The compensation is providedby the fact that the discharge of capacitor El i ceases when thepotential of the anode of rectifier I635 equals that of its cathode; andthe potential of such cathode is fixed by the amount of current flowingthrough resistors 99 and 5% in the cathode lead of tube 98. Thus whenthe grid of tube $8 is made more negative at the termination of acurrent pulse through tube E5, the current flowing through the cathoderesistors 9t and mil is decreased, thereby making the potential of thecathode of tube use more negative by the amount necessary to fix thestate of charge on capacitor 2% at a value corresponding to theincreased negative potential applied to the anode of rectifier Hit. Theconnection of the anode of rectifier 1% to the resistors 29 and ltdsimilarly applies the necessary compensation for the efiect of thechange in potential of capacitor we on the discharge voltage ofcapacitor 96.

It will be noted that by reason of the fact that the maintenance 'of analtered potential across the capacitor 109 is not dependent upon acontinued deviation in frequency of the source i from that of the source2, the regulating system I have above described is not one which merelytends to react to minimize deviation in frequency of source I from thatof source 2; it is one which causes corrective action to continue untilall deviation disappears, and the corrective action then ceases untilanother deviation in frequency occurs.

I claim as my invention:

1. In combination with a first alternatingcurrent source and a secondalternating-current source likely to deviate in frequency, frequencydetermining means for one source the regulating action ofwhich-isresponsive to a direct-current voltage, means for obtaining from saidfirst and second alternating current sources a first voltage and asecond voltage having a frequency equal to the difference-frequency ofsaid sources, said first voltage leading said second voltage in phasewhen the frequency of said first source exceeds that of said secondsource and lagging said second voltage when the frequency of said firstsource is less than that of said second source, means for obtaining'twoauxiliary voltages of opposite phase from each other by squaring anddifferentiating one of said voltages, means for superimposing one ofsaid auxiliary voltages on the other of said beat frequency voltages tocontrol current flow through a first channel, means for superimposingthe other auxiliary voltage on said other beat frequency voltage tocontrol current flow through a second channel, means for causing currentflow in said first channel to charge a capacitor with a definitequantity of electricity for each cycle of beat frequency, and means forcausing current flow in said second channel to discharge from saidcapacitor a definite quantity of electricity for each cycle of said beatfrequency, and means for controlling said frequency regulating means inresponse to the voltage of said condenser.

2. In combination with a first alternating-current source and a secondalternating-current source likely to deviate in frequency, means forderiving from said sources a first beat fr quency voltage and a secondbeat frequency voltage, the phase of said first beat frequency voltagerelative to said second beat frequency voltage changing in sign when thedifference in frequency of said first source from said second sourcechanges sign, means for squaring and differentiating the first beatfrequency, means for superimposing in a first channel said second beatfrequenc and the output of said diiferentiator, means for superimposingin a second channel said second beat frequency and the negative of theoutput voltage of said difierentiator, a saturated amplifier in saidfirst channel rendered conductive only when said superposed voltagestherein exceed a critical value, a saturated amplifier in said secondchannel rendered conductive only when the superposed voltages thereinexceed said critical value, a capacitor in said first channel chargedwhen the amplifier therein is conductive, a capacitor in said secondchannel charged only when the amplifier therein is rendered conductive,and means for causing the capacitor in said first channel to charge anauxiliary capacitor, and means in said second channel for causing thecapacitor therein to discharge said auxiliary capacitor.

3. Incombination with a first voltage source and a second voltage sourcelikely to deviate therefrom in frequency, 'a first channel traversed bycurrent'pulses only when the frequency of said first source exceeds thatof said second source, a second channel traversed by current pulses onlywhen the frequency of said first source is less than that of said secondsource, the frequency of said pulses being equal to thedifference-frequency of said sources, means for charging a capacitorwith constant increments of positive voltage for each current pulse insaid first channel, and means for charging said capacitor with constantincrements of negative voltage equal in magnitude to said positivevoltage for each pulse in said second channel.

4. The method of maintaining frequency equality between two alternatingvoltage sources which comprises deriving from said voltage sources afirst beat frequency voltage and a second beat frequency voltage havinga phase difference from said first beat frequency voltage which chan essi n when the difference frequency of said sources passes through zero,squaring and differentiating said first beat fre uency volta e, superimosing the result of said differentiation on said second beat frequencyvoltage positively in one current channel and negatively in the secondcurrent channel, causing current pulses in said first current channel toincrease a control voltage by constant positive amounts for each cycleof beat frequency, and causing current pulses in said second channel todecrease said control voltage by equal amounts for each cycle of saidbeat frequency, and controlling the fre uency of one of said sources bysaid control voltage.

5. In combination with a first alternating-current source and a secondalternating-current source, means for supplying a first mixer tube witha first pair of currents derived, respectively, from said two sources,means for supplying a second mixer tube with a second pair of currentsderived, resnectively, from said two sources, the phase differencebetween said first pair of current being 90 different from the phasedifference between said second pair of currents, means for squaring anddifferentiating the output of said first mixer tube to obtain a firstpulse voltage and a second pulse voltage of opposite phase to said firstpulse voltage, means for superimposing in one current channel said firstpulse voltage and the output of said second mixer tube, means forsuperimposing in a second current channel the output of said pulsevoltage and the output of said second mixer tube, the currents in saidchannels comprising current pulses, means for causing a first capacitorto acquire a definite charge for each pulse in said first channel and toimpart a definite negative charge to an auxiliary capacitor, means forcausing a second capacitor to acquire equal charge and to impart anequal negative charge to said auxiliary capacitor for each pulse in saidsecond channel.

6. The method of balancing the frequencies of a firstalternating-current source with a second alternating-current sourcewhich comprises mixing a current from said first alternating-currentsource with a first current derived from said second alternating-currentsource to produce a first output current of the difierence-frequency ofsaid sources, mixing a current from said first alternating-currentsource with a second current derived from said secondalternating-current source which differs by substantially 90 in phasefrom said first current to produce a second output current having afrequency equal to the differencefrequency of said sources, squaring anddifferentiating said first output current to produce a firstpulsed-current having said difference-frequency and also a secondpulsed-current having said difference-frequency, superimposing saidfirst pulsed-current and said second output current to produce a firstpulsed-control voltage, superimposing said second pulsed-current andsaid second output current to produce a second pulsed-control voltage,said control voltages having the difference frequency of said sources,and increasing a frequency-control voltage for one of said sources by adefinite positive amount for each pulse of said pulsed-control voltageand decreasing said frequency-control voltage by the same definiteamount for each pulse of said second pulsed-control voltage.

7. In combination with a first alternating-current source and a secondalterating-current source likely to deviate in frequency, means forobtaining from said sources a first voltage and a second voltage havinga frequency equal to the difference-frequency of said sources, saidfirst voltage leading said second voltage in phase when the frequency ofsaid first source exceeds that of said second source and lagging saidsecond volt age when the frequency of said first source is less thanthat of said second source, means for obtaining two auxiliary voltagesof opposite phase from each other by squaring and differentiating one ofsaid voltages, means for superimposing one of said auxiliary voltages onthe other of said beat frequency voltages to control current flowthrough a first channel, means for superimposing the other auxiliaryvoltage on said other beat frequency voltage to control current flowthrough a second channel, means for causing current flow in said firstchannel to charge a capacitor with a definite quantity of electricityfor each cycle of beat frequency, and means for causing current flow insaid second channel to discharge from said capacitor a definite quantityof electricity for each cycle of said beat frequency, and means forcausing current flow in said second channel to discharge from saidcapacitor a definite quantity of electricity for each cycle of said beatfrequency.

8. The method of maintaining frequency equality between two alternatingvoltage sources which comprises deriving from said voltage sources afirst beat frequency voltage and a second beat frequency voltage havinga phase difference from said first beat frequency voltage which changessign when the difference frequency of said sources passes through zero,squaring and differentiating said first beat frequency voltage,superimposing the result of said differentiation on said second beatfrequency voltage positively in one current channel and negatively inthe second current channel, causing current pulses in said first currentchannel to increase a control voltage by constant positive amounts foreach cycle of beat frequency, and causing current pulses in said secondchannel to decrease said control voltage by equal amounts for each cycleof said beat frequency.

JOHN R. BOYKIN.

REFERENCES CITED The following referenlces are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,944,315 Clapp Jan. 23, 19342,176,742 LaPierre Oct. 1'7, 1939 2,337,328 Hathaway Dec. 21, 19432,406,309 Zeigler et al Aug. 20, 1946

